The use of light emitting diodes in general lighting is still limited due to limitations in light output level or flux generated by the illumination devices. Illumination devices that use LEDs also typically suffer from poor color quality characterized by color point instability. The color point instability varies over time as well as from part to part. Poor color quality is also characterized by poor color rendering, which is due to the spectrum produced by the LED light sources having bands with no or little power. Further, illumination devices that use LEDs typically have spatial and/or angular variations in the color. Additionally, illumination devices that use LEDs are expensive due to, among other things, the necessity of required color control electronics and/or sensors to maintain the color point of the light source or using only a small selection of produced LEDs that meet the color and/or flux requirements for the application.
Illumination devices that use LEDs also typically suffer from poor dimming characteristics, particularly at low light output levels. This is commonly referred to as deep dimming. Constant current reduction (CCR) dimming control schemes are limited in their ability to achieve deep dimming due to LED driver limitations. In addition, operation of LEDs at current levels below approximately 10% of their rated current level may lead to operational and reliability difficulties. Thus, constant current reduction dimming control schemes are typically limited to no less than 10% of the normal, undimmed light output. Digital dimming techniques are also employed. In one example, pulse width modulated (PWM) dimming control schemes are employed. In a pulse width modulated control scheme, the current supplied to the LED is switched on and off at a fixed frequency, and the current output is modulated by adjusting the duty cycle of the on pulse. At dimming levels below, for example, 5%, pulse width modulated dimming schemes typically exhibit unsmooth transitions between each digital dimming step. At very small duty cycles, limitations in digital resolution cause relatively large jumps in duty cycle at each digital dimming step. For example, when adjusting the duty cycle by 1% to dim a light from 100% of the full intensity to 99% of the full intensity, the relative change in intensity is small. However, when adjusting the duty cycle by 1% to dim a light from 10% of the full intensity to 9% of the full intensity, the relative change in intensity is very large, 10%. These relatively large jumps manifest themselves as jumps in light output that are clearly visible when the light output is changed at low light levels.
In order for PWM to produce smooth transitions between each digital dimming step at low intensities, a large number of pulses are required in a period. To produce a large number of pulses, however, requires either high clock frequencies, which increases costs, or a large PWM period, which results in an undesirable visible flicker.
Consequently, improvements to illumination device that uses light emitting diodes as the light source are desired. In particular, improvements in deep dimming performance are desired.